In the fabrication of semiconductor devices, isolation structures are formed between active areas in which electrical devices, such as transistors, memory cells or the like, are to be formed. The isolation structures are typically formed during initial processing of a semiconductor substrate, prior to the formation of electrical active devices. Typical isolation techniques include shallow trench isolation (STI), which may enable an active area with high density. In addition, the flatness of the resulting wafer enables more precise pattern definition for subsequent layers.
Shallow trench isolation (STI) techniques involve the formation of shallow trenches, which may then be filled with dielectric material, such as silicon dioxide, to provide electrical isolation. During the formation of shallow trenches, trench depth control across a wafer is critical in trench etching because the trench depth uniformity determines the uniformity of the fill and of the following planarization processes such as CMP (i.e., chemical-mechanical polishing). More importantly, as feature sizes decrease, precise control of the trench depth becomes even more critical for device performance.
One conventional technique for characterizing the trench depth is scanning electron microscope (SEM) cross section analysis. However, this conventional technique has drawbacks. For example, in order to do the cross section analysis, the wafer sample has to be destroyed to expose the cross section of the sample. In addition, the SEM analysis is time-consuming with turnaround times measured in days, which significantly delays diagnosing process issues. Further, SEM resolution presents a problem for repeatable depth measurement of submicron features.
A second conventional technique for characterizing the trench depth is stylus profilometry, which is a non-destructive method. In this method, step-height measurements are conducted on test structures to monitor trench depth before filling the trench. However, this method also has drawbacks. For example, one drawback is caused by the lack of high aspect ratio capability and high stylus forces that prevent within-die measurements.
A third conventional technique for characterizing the trench depth is in-line profilometry, such as an atomic force profilometry (AFP) technique, which is a non-destructive in-line monitor for smaller STI structures. The in-line profilometry technique may be used in a production environment. However, it also has drawbacks. For example, one drawback is that only limited data can be collected. To understand the process uniformity, it is often desired to collect statistical data for a wafer and/or further make a parametric correlation with device performance while monitoring the trench depth. However, in-line data is in general very limited and insufficient for this.